Box 7:Exotic species introduced via ballast-water transport have displaced natural fauna and flora and may be threatening the world's coastal ocean biota.

BIOLOGICAL INVASIONS VIA BALLAST-WATER TRANSPORT

The most important global dispersal mechanism for passively moving shallow-water organisms between and across oceans is ships' ballast water and sediments. One estimate suggests that more than 3,000 species of coastal marine animals and plants are in transit around the world at any given moment in the ballast of ships. The result is that aquatic habitats all over the world are becoming dominated by exotic species. Scores if not hundreds of invasions have occurred during and since the 1980s alone. Examples include:

The invasion of the Black and Azov Seas by the carnivorous American comb jellyfish Mnemiopsis leidyi, resulting in plankton biomass declines of as much as 90 percent and a startling decline in the anchovy fishery.

The invasion of San Francisco Bay by the Chinese clam Potamocorbula amurensis (see Box 6).

The invasion of the Texas coastline by the Indo-Pacific mussel Perna perna, which now forms monocultural stands on jetties for scores of kilometers along the Gulf of Mexico shore.

The invasion of the Australian coast by Japanese red-tide dinoflagellates and by the carnivorous, abalone-eating Japanese starfish Asteria amurensis.

The invasion of the Great Lakes by fouling zebra mussels (leading to economic losses of hundreds of millions of dollars per year), by three species of fish, and by a carnivorous water flea, all from European waters.

Introductions occur in all habitats, although they appear to be most common in estuaries and bays. Many invasions have profoundly altered the distribution and abundance of native species and the food webs of the systems they have invaded. Human introductions transcend natural dispersal barriers, bringing into contact organisms with no evolutionary experience between them, thus setting the stage for often dire results. The prospects for future spectacular invasions of coastal waters around the world remain extraordinarily high, as long as ballast water continues to be moved and released.

and benthic organisms (Jokiel, 1980; Jokiel and York, 1984; Kramer, 1990; Bothwell et al., 1994) from UV-B can and does occur in relatively shallow or surface waters. Recent work has further documented the, effects of increased UV-B on corals (Gleason and Wellington, 1994). The species-specificity of Antarctic phytoplankton susceptibility to UV-B damage, for example,

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